The disclosure herein relates to treatment fluids employed in subterranean operations, and more particularly, to the stabilization of heteropolysaccharide-based gels in brine-based treatment fluids as part of subterranean operations, such as gravel packing and combined gravel packing-fracturing operations.
Gravel packing treatments have been used to reduce the migration of unconsolidated formation particulates into a well bore. During gravel packing operations, gravel particulates are generally carried to a well bore by a hydrocarbon or aqueous carrier fluid. The gravel particulates are suspended in a carrier fluid, which may be viscosified, and the carrier fluid is pumped into a well bore in which the gravel pack is to be placed. The carrier fluid may leak off into the subterranean zone and/or return to the surface while the particulates are left in the zone. The resultant gravel pack may act as a filter to separate formation sands from produced fluids while permitting the produced fluids to flow into the well bore. A portion of the gravel may be coated with tackifying agents, among other things, to further help control the migration of formation fines. Typically, gravel pack operations involve placing a gravel pack screen in the well bore and packing the surrounding annulus between the screen and the well bore with gravel designed to prevent the passage of formation sands through the pack. Such gravel packs may be used to stabilize the formation while causing minimal impairment to well productivity.
For gravel packing operations requiring higher densities, heteropolysaccharide-based viscosifiers, such as diutan gels are commonly employed in brine-based fluids. However, when diutan gels are prepared in high salt concentration brines (e.g., brine density greater than about 10 ppg such as, but not limited to, sodium bromide, calcium bromide, calcium chloride, cesium formate, cesium acetate, potassium formate and potassium acetate) and temperatures above about 180° F. (about 82° C.), diutan exhibits a salting-out effect at pH less than about 3. The salting-out effect is believed to be a cumulative effect of the following factors: temperature, brine density, concentration of diutan and pH. Hence, to maintain the requisite rheological properties of the treatment fluids, among the required considerations are a careful selection of salts and salt concentrations. In general, the salting-out effect trends along the Hofmeister series observed for proteins. Thus, bromides and iodides relative to chlorides are generally more favorable for preventing heteropolysaccharide salting-out. Iodides in particular, while providing good prevention of salting out, are not practical in typical gravel packing operations due to the prohibitive cost of iodide salts. Thus, the salting-out effect tends to limit the brine types, brine densities, and temperatures in which the treatment fluids can be prepared and used.
Other solutions to prevent or reduce salting-out include the use of urea, which has also not proved to be commercially viable. Other avenues employing, for example, xanthan-based gelling agents in lieu of diutan suffer from incompatibilities with crosslinkers used in xanthan-based systems as well as temperature limitations in operation.